Redox grafting of diazotated anthraquinone as a means of forming thick conducting organic films

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

Direct In Situ Spectroscopic Evidence for Solution-Mediated Oxygen Reduction Reaction Intermediates in Aprotic Lithium-Oxygen Batteries

A fundamental understanding of the reaction process is essential to predict and enhance the performance of electrochemical devices. As a central reaction in aprotic lithium-oxygen (Li-O₂) batteries, the oxygen reduction reaction (ORR) has been confronted with the "sudden-death" phenomenon caused by the cathode passivation from discharge product Li₂O₂. The soluble catalyst (e.g., reduction mediator) promoted solution-mediated ORR represents an elegant solution. However, no direct molecular evidence is available so far, and its link to Li-O₂ batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct spectroscopic evidence (i.e., LiAQ and LiAQO₂) of the solution-mediated ORR on a model anthraquinone (AQ, a typical reduction mediator)-immobilized Au electrode. With the assistance of density functional theory calculations and differential electrochemical mass spectrometry, the related elementary reaction steps of the solution-mediated ORR are proposed. This work provides intuitive insights into the AQ-catalyzed solution-mediated ORR mechanism that is helpful in the optimization and tailor-design of soluble catalysts for excellent next-generation Li-O₂ batteries.

Nanostructured Mixed Layers of Organic Materials Obtained by Nanosphere Lithography and Electrochemical Reduction of Aryldiazonium Salts

In this work, we have combined nanosphere lithography with electrochemical reduction of aryldiazonium salts to elaborate nanostructured mixed layers of organic materials. The strategy consists first in the deposition of a close-packed hexagonal monolayer of microbeads used as a mask for the electroreduction of a first aryldiazonium salt. After removing the beads, an ultrathin organic layer with holes remains. Then, a second aryldiazonium salt is electrochemically reduced selectively inside the holes. The relative thickness of the two deposited materials can be changed, leading to mixed layers of different topographies. Moreover, using diazoniums with complementary redox properties, a modified bifunctional electrode acting as a filter for electron transfer with a low potential gap has been obtained. Such layers are similar to low-band-gap organic semiconductors that can be easily n or p doped. Despite this analogy, the oxidation and reduction of redox probes in solution on such nanostructured surfaces occur on completely separated areas of the mixed layer.

Promoting Solution Discharge of Li-O2 Batteries with Immobilized Redox Mediators

For years, the aprotic Li-O₂ battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery. Recent fundamental study of Li-O₂ electrochemistry revealed that this dilemma is caused by the growth of Li₂O₂ on the cathode surface and can be solved by discharging Li₂O₂ in the electrolyte solution. Among the strategies that can promote solution growth of Li₂O₂, redox mediators (i.e., soluble catalysts) demonstrate prominent performance. However, soluble redox mediators may shuttle from the cathode to the lithium anode and decompose thereon, causing severe deterioration of the lithium anode and degradation of the mediators' functionality. Here, we report that immobilized redox mediators (e.g., anthraquinone, AQ) in the form of a thin conductive polymer film (PAQ) on the cathode can effectively promote solution growth of Li₂O₂ even in weakly solvating electrolyte solutions that would otherwise lead to surface film growth and early cell death. The PAQ-catalyzed Li-O₂ battery can deliver a discharge capacity that is up to ∼50 times what its pristine counterpart does at the same current densities and is comparable to the capacity realized by soluble AQ-catalyzed Li-O₂ batteries. Most importantly, the adverse "cross-talk" between the lithium anode and the redox mediators immobilized on the cathode has been completely eliminated.

Molecular Sieving and Current Rectification Properties of Thin Organic Films

For the purpose of preparing well-organized functional surfaces, carbon and gold substrates were modified using electroreduction of a tetrahedral-shape preorganized tetra-aryldiazonium salt, leading to the deposition of ultrathin organic films. Characterization of the modified surfaces has been performed using cyclic voltammetry, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements. The specific design of the tetra-aryldiazonium salts leads to an intrinsic structuring of the resulting organic films, allowing molecular sieving and current rectification properties toward redox probes in solution.

Electroreduction of Viologen Phenyl Diazonium Salts as a Strategy To Control Viologen Coverage on Electrodes

A majority of the reported electrografting of aryldiazonium salts result in the formation of covalently attached films with a limited surface coverage of below 5 nmol·cm^-2. Herein, we report the preparation of higher-thickness redox-active viologen-grafted electrodes from the electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies were successfully applied for various conductive surfaces, including glassy carbon (GC), gold disc, indium tin oxide glass, mesoporous TiO₂ electrodes, and 3D compacted carbon fibers. A robust maximal viologen coverage, Γ_viologen = 9.5 nmol·cm^-2, was achieved on a GC electrode by CV electroreduction. Electroreduction held at a fixed potential at E_appl. = -0.3 V can fabricate viologen-grafted electrodes with Γ_viologen in the range of 0-37 nmol·cm^-2 in a controllable way, by simply adjusting the electrodeposition time t_appl.. Time-dependent Γ_viologen were found to be 10 nmol·cm^-2@2 min, 20 nmol·cm^-2@4.2 min, and 30 nmol·cm^-2@7 min. Furthermore, a TiO₂ electrode coupled with Γ_viologen of 140 nmol·cm^-2 exhibited electrochromic performance, with the color changing from pale yellow to blue and red brown.

Robust All-Carbon Molecular Junctions on Flexible or Semi-Transparent Substrates Using "Process-Friendly" Fabrication

Large area molecular junctions were fabricated on electron-beam deposited carbon (eC) surfaces with molecular layers in the range of 2-5.5 nm between conducting, amorphous carbon contacts. Incorporating eC as an interconnect between Au and the molecular layer improves substrate roughness, prevents electromigration and uses well-known electrochemistry to form a covalent C-C bond to the molecular layer. Au/eC/anthraquinone/eC/Au junctions were fabricated on Si/SiOx with high yield and reproducibility and were unchanged by 10(7) current-voltage cycles and temperatures between 80 and 450 K. Au/eC/AQ/eC/Au devices fabricated on plastic films were unchanged by 10(7) current density vs bias voltage (J-V) cycles and repeated bending of the entire assembled junction. The low sheet resistance of Au/eC substrates permitted junctions with sufficiently transparent electrodes to conduct Raman or UV-vis absorption spectroscopy in either reflection or transmission geometries. Lithographic patterning of Au/eC substrates permitted wafer-scale integration yielding 500 devices on 20 chips on a 100 mm diameter wafer. Collectively, eC on Au provides a platform for fabrication and operation of chemically stable, optically and electrically functional molecules on rigid or flexible materials. The relative ease of processing and the robustness of molecular junctions incorporating eC layers should help address the challenge of economic fabrication of practical, flexible molecular junctions for a potentially wide range of applications.

Metabolic Activation of Rhein: Insights into the Potential Toxicity Induced by Rhein-Containing Herbs

Rhein is a major component of the many medicinal herbs such as rhubarb. Despite wide use, intoxication cases associated with rhein-containing herbs are often reported. The present work aimed to investigate if rhein was subject to metabolic activation leading to toxicity. Upon incubations with different species of liver microsomes, three monoglucuronides were identified, corresponding to two hydroxyl glucuronides and one acyl glucuronide via the carboxyl group, respectively. Further study revealed that rhein acyl glucuronide was chemically reactive, and showed cytotoxicity toward hepatocarcinoma cells. In addition, significant species differences in glucuronidation of rhein were observed between laboratory animals and humans. Reaction phenotyping experiments demonstrated that rhein acyl glucuronide was catalyzed predominantly by uridine 5'-diphospho-glucuronosyltransferase 1A1, 1A9, and 2B7. Taken together, the present study confirmed that rhein could be metabolically activated via the formation of acyl glucuronide, especially in human.

Functionalizing Arrays of Transferred Monolayer Graphene on Insulating Surfaces by Bipolar Electrochemistry

Development of versatile methods for graphene functionalization is necessary before use in applications such as composites or as catalyst support. In this study, bipolar electrochemistry is used as a wireless functionalization method to graft 4-bromobenzenediazonium on large (10 × 10 mm(2)) monolayer graphene sheets supported on SiO2. Using this technique, transferred graphene can be electrochemically functionalized without the need of a metal support or the deposition of physical contacts. X-ray photoelectron spectroscopy and Raman spectroscopy are used to map the chemical changes and modifications of graphene across the individual sheets. Interestingly, the defect density is similar between samples, independent of driving potential, whereas the grafting density is increased upon increasing the driving potential. It is observed that the 2D nature of the electrode influences the electrochemistry and stability of the electrode compared to conventional electrografting using a three-electrode setup. On one side, the graphene will be blocked by the attached organic film, but the conductivity is also altered upon functionalization, which makes the graphene electrode different from a normal metal electrode. Furthermore, it is shown that it is possible to simultaneously modify an array of many small graphene electrodes (1 × 1 mm(2)) on SiO2.

Probing electron-phonon excitations in molecular junctions by quantum interference

Electron-phonon coupling is a fundamental inelastic interaction in condensed matter and in molecules. Here we probe phonon excitations using quantum interference in electron transport occurring in short chains of anthraquinone based molecular junctions. By studying the dependence of molecular junction’s conductance as a function of bias voltage and temperature, we show that inelastic scattering of electrons by phonons can be detected as features in conductance resulting from quenching of quantum interference. Our results are in agreement with density functional theory calculations and are well described by a generic two-site model in the framework of non-equilibrium Green’s functions formalism. The importance of the observed inelastic contribution to the current opens up new ways for exploring coherent electron transport through molecular devices.

Electrochemical properties of gold and glassy carbon electrodes electrografted with an anthraquinone diazonium compound using the rotating disc electrode method

The RDE method was combined with the electrografting procedure to prepare thick AQ films on Au and glassy carbon electrodes.

Control of the grafting of hybrid polyoxometalates on metal and carbon surfaces: toward submonolayers

A Keggin-type POM is attached to gold or glassy carbon surfaces by electro(chemical) or peptidic coupling. In addition to demonstrating the robust attachment of the POMs (by electrochemistry, XPS, and IRRAS), the surface concentration, layer thickness, and rate constant for electron transfer from the surface to the POMs have been measured. The use of such complementary techniques is mandatory to characterize the modified electrodes properly. Whatever the grafting method, experimental conditions are found to allow monolayer or submonolayer coverage. Besides covalently grafted species, additional electrostatically bonded POMs are present in the film. Cathodic polarization allows removing them to get a grafted film that is stable with time and potential, which is a requirement in the design of molecular memories.

On electrogenerated acid-facilitated electrografting of aryltriazenes to create well-defined aryl-tethered films

The mechanism of electrogenerated acid-facilitated electrografting (EGAFE) of the aryltriazene, 4-(3,3-dimethyltriaz-1-enyl)benzyl-1-ferrocene carboxylate, was studied in detail using electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry. The measurements support the previously suggested mechanism that electrochemical oxidation of the EGA agent (i.e., N,N'-diphenylhydrazine) occurs on the forward oxidative sweep to generate protons, which in turn protonate the aryltriazene to form the corresponding aryldiazonium salt close to the electrode surface. On the reverse sweep, the electrochemical reduction of the aryldiazonium salt takes place, resulting in the electrografting of aryl groups. The EGAFE-generated film consists of a densely packed layer of ferrocenyl groups with nearly ideal electrochemical properties. The uncharged grafted film contains no solvent and electrolyte, but counterions and solvent can easily enter and be accommodated in the film upon charging. It is shown that all ferrocene moieties present in the multilayered film are electrochemically active, suggesting that the carbon skeleton possesses a sufficiently high flexibility to allow the occurrence of fast electron transfers between the randomly located redox stations. In comparison, EQCM measurements on aryldiazonium-grafted films reveal that they have a substantially smaller electrolyte uptake during charging and that they contain only 50% electroactive ferrocenyl groups relative to weight. Hence, half of these films consist of entrapped supporting electrolyte/solvent and/or simply electrochemically inactive material due to solvent inaccessibility.

Electrochemical modification of gold electrodes with azobenzene derivatives by diazonium reduction

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X-ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi-reversible voltammogram of the Fe(CN)6 (3-/4-) redox couple on bare Au and a sigmoidal shape for the GBC- and FBK-modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)6 (3-/4-) response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)6 (3-/4-) redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge-transfer rates to the Fe(CN)6 (3-) probe for the GBC- and FBK-modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.

Controlling formation of single-molecule junctions by electrochemical reduction of diazonium terminal groups

We report controlling the formation of single-molecule junctions by means of electrochemically reducing two axialdiazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in situ between the molecule and gold electrodes. We report a yield enhancement in molecular junction formation as the electrochemical potential of both junction electrodes approach the reduction potential of the diazonium terminal groups. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond.

Elucidation of the mechanism of redox grafting of diazotated anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.